Steroid hormones regulate target cell function by binding to cytoplasmic receptor proteins. These interact with acceptor sites on nuclear chromatin. There they induce messenger RNAs for specific cellular proteins which are responsible for alterations in endocrine physiology. Progesterone has been difficult to study at the molecular level in mammalian systems. We have therefore characterized the progesterone-sensitive chick oviduct over the past 8 years and have shown it to be a representative model of steroid hormone action. Major insights into sex steroid function have been possible through advances in three areas: hormone receptors, chromatin structure and transcription, and analysis of specific gene products. We plan an integrated series of studies in these areas, monitoring specific gene products from interactions of pure hormone-receptor complexes with chromatin in vitro. Our approach will be: Preparation of large quantities of homogeneous intact receptors and their individual subunits: a) preparation of antibodies to defined receptor components; b) studies on receptor subunit interactions; c) structural analysis of receptor proteins and their hormone binding sites; and d) use of receptor subunits as affinity probes for isolating components of the "acceptor sites" in hormone-responsive chromatin. Construction of a cell-free system composed of purified components for studying the molecular action of steroid hormone receptors: a) reconstitution of a chromatin transcription system responding to steroid hormone receptors; b) synthesis of hybridization probes to measure expression of selected hormone-responsive genes; and c) construction of "minichromosomes" containing single hormone responsive genes. This work should afford us an opportunity to define precisely the molecular basis of hormone-mediated gene expression, resulting in an advance in our understanding of endocrine physiology, reproductive biology and steroid hormone action.